CALCIFIED MICROBES IN NEOPROTEROZOIC CARBONATES - IMPLICATIONS FOR OUR UNDERSTANDING OF THE PROTEROZOIC CAMBRIAN TRANSITION
CALCIFIED MICROBES IN NEOPROTEROZOIC CARBONATES - IMPLICATIONS FOR OUR UNDERSTANDING OF THE PROTEROZOIC CAMBRIAN TRANSITION
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DOI：
10.2307/3515029
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发表时间：
1993-12-01
期刊：
影响因子：
1.6
通讯作者：
SWETT, K
中科院分区：
文献类型：
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作者：
KNOLL, AH;FAIRCHILD, IJ;SWETT, K
斯匹次卑尔根岛新元古代德拉肯组潮滩和泻湖白云岩中保存完好的碳酸盐结构，包括严重钙化的微化石。在轻度蒸发的潮间带至潮上环境中，形成壳的蓝藻被沉积在鞘上和鞘内的碳酸盐局部保存。相反，柱状叠层石中的钙化细丝反映了潮下降水。白云岩中的长丝模独立地记录了极早的岩化作用。在德拉肯和其他元古代碳酸盐中存在严重钙化的蓝藻，限制了对元古代-寒武纪边界附近广泛出现钙化微生物的潜在解释。我们认为元古代样品的稀有主要反映了海底或水柱中碳酸盐晶体的丰富和广泛分布。蓝藻鞘只有在环质和/或文石核缺失的地方才能有效地竞争碳酸盐成核和生长的位置。在这种观点下，钙化微化石的元古宙-寒武纪扩张主要反映了骨骼作为碳酸盐沉积的主要介质的出现。
Tidal flat and lagoonal dolostones of the Neoproterozoic Draken Formation, Spitsbergen, exhibit excellent preservation of carbonate fabrics, including heavily calcified microfossils. The crust-forming cyanobacterium Polybessurus is preserved locally by carbonate precipitated on and within sheaths in mildly evaporitic upper intertidal to supratidal environments. In contrast, calcified filaments in columnar stromatolites reflect subtidal precipitation. Filament molds in dolomicrites independently document extremely early lithification. The presence of heavily calcified cyanobacteria in Draken and other Proterozoic carbonates constrains potential explanations for the widespread appearance of calcified microorganisms near the Proterozoic-Cambrian boundary. We propose that the rarity of Proterozoic examples principally reflects the abundance and wide distribution of carbonate crystals precipitated on the sea floor or in the water column. Cyanobacterial sheaths would have competed effectively as sites for carbonate nucleation and growth only where cylcitic and/or aragonitic nuclei were absent. In this view, the Proterozoic-Cambrian expansion of calcified microfossils primarily reflects the emergence of skeletons as principal agents of carbonate deposition.